1. Field of the Invention
This invention relates generally to articles having an electrodeposited metallic layer and, more particularly, to articles having an electrodeposited layer of ruthenium and to processes of producing such articles.
2. Description of the Prior Art
A metallic layer is electrodeposited on many types of articles to improve either their esthetic or functional properties. In many applications, such as electrical contacts, a hard and wear resistant metallic layer greatly improves the functional properties of the article. Use of ruthenium in such applications is theoretically attractive because it is very hard and is relatively cheap. For example, plated ruthenium is potentially harder, more durable and cheaper than the plated gold commonly used on electrical contacts such as those on the magnetic reeds of sealed reed switches. In spite of these attractive qualities, ruthenium has not received widespread electroplating use because of difficulties encountered in plating moderately thick ruthenium layers that are not brittle or prone to crack when subjected to stresses such as those encountered in the sealing of the envelopes of sealed reed switches.
The plating solution that has appeared to yield the best quality ruthenium plating, while maintaining reasonable plating efficiencies, uses the ammonium, lithium, potassium or sodium salt of the nitrogen bridged anionic complex [Ru.sub.2 N(H.sub.2 O).sub.2 Cl.sub.8 ].sup.3-. Methods directed to preparing and using the complex for electroplating are described in U.S. Pat. No. 3,576,724. Other processes, such as described in U.S. Pat. Nos. 2,057,638 and 3,122,544, may produce this or a similar nitrogen bridged binuclear ruthenium complex. The preparatory method of U.S. Pat. No. 3,576,724 heats an aqueous solution of ruthenium chloride, in either solid hydrated form or in a hydrochloric acid solution, and excess sulphamic acid for a period of time, typically 48 hours, sufficient to hydrolyze the sulphamate. The ammonium salt is obtained from the resulting solution, for use either in a plating bath or as the starting material in a conventional ion exchange process yielding the potassium, lithium or sodium salt, by concentrating the solution, cooling to 0.degree. C and filtering. According to U.S. Pat. No. 3,576,724, the amount of sulphamic acid should be between 4 and 9 molecular proportions of sulphamic acid for each atomic proportion of ruthenium. The preferred molar ratio of sulphamic acid to ruthenium is stated to be between 4 and 5 for the best yield. Hydrochloric acid may be added to the ruthenium chloride and sulphamic acid mixture, after completion of the initial reaction, to obtain the best yield of the desired ruthenium complex by preventing undesired groups, such as bisulphate groups, from entering the complex and replacing chlorine atoms.
A particular problem not satisfactorily overcome by prior art plating baths is oxidation of ruthenium at the anode to states higher than the +4 state in the ruthenium complex. Such oxidation products degrade the plating current efficiency and may affect plating quality adversely. Oxidation may continue to the +8 or RuO.sub.4 state which has an appreciable vapor pressure and is poisonous. This problem has been attacked by adding a reducing agent, such as ammonium formate or ammonium sulphamate to the plating bath. However, these reducing agents, as well as the buffering materials used, often decrease the quality of the plating, possibly because of impurities plated with the ruthenium, and may prevent ruthenium plating entirely.
Ruthenium platings made in accordance with the described prior art are not satisfactory for many purposes because they tend to crack if the plating thickness exceeds 0.3 .mu.m and is subjected to thermal shock such as that typically accompanying the sealing of reed switches.